The invention is directed to solar simulators and more particularly to a high efficiency solar simulators employing flexible optic technology to transfer simulated sunlight from a high intensity source to an enclosure and illuminating a test specimen with the light from the source either by flood illuminating the test specimen or by first collimating the light and then directing the collimated light on to the specimen.
The total radiation of the sun, as well as its spectrum, have been subjects of much investigation over the centuries. With the advent of space flight and the availability of better instrumentation in the past few years, very accurate measurements of solar characteristics have been recorded. Critical examination of numerous recorded high-altitude measurements of the sun's spectral radiance have been documented. These measurements have been analyzed and as a result of the analysis a spectral standard table has been established. This table is commonly referred to as The Air-Mass Zero Solar Spectrum. This table provides a close simulation of sunlight near the earth but outside of the earth's atmosphere.
The earth's atmosphere creates substantial loss due to absorption and scattering throughout the spectrum. The absorption is particularly strong in the shortwave ultraviolet, due primarily to ozone; and in the longer-wave infrared, due to water vapor and carbon dioxide. The lower the elevation of the sun, and the greater the optical length through the atmosphere, the greater the absorption, especially in the ultraviolet.
A solar spectrum with the sun at zenith (90 degrees elevation) shows minimum absorption and is referred to as an Air-Mass One Spectrum. At an elevation of 30 degrees (60 degrees from zenith), the path length is doubled and the solar spectrum is called Air-Mass Two.
Since atmospheric scattering and absorption constituents are continually changing, measurements of solar radiance on the earth's surface necessarily lack reproducibility and accuracy. Models have been developed that start with spectral distribution outside the atmosphere (Air-Mass Zero) and calculate the effects of scattering and absorption for assumed atmospheric conditions and sun angles. It is the spectral radiance of a family of Air-Mass distribution values that solar simulators are designed to simulate.
Ideally, the solar simulator would match the entire spectrum or the sun, both in spectral distribution and amplitude, but this rigorous requirement is not always necessary. Different applications make some portions of the spectrum more important than others. The rendition of colors is important mainly within the visible portion of the spectrum. Testing of dye and pigment fading, and of biological effects emphasizes the ultraviolet. Photographic standardization is in the visible and near-infrared, while simulators for solar heating require the addition of longer wavelength infrared energy. Space technological testing requires the full spectrum. In some cases, the power required is many times the solar constant, and in other cases, spatial uniformity and some degree of collimation are more important.
All simulators employ a light source, some collecting and projecting optics and (usually) filtering to provide the spectrum required. No light source exactly duplicates the spectral radiance of the sun as seen on the earth. Ninety Percent of the solar energy is distributed between 276 and 4960 nm. Over this region, the sun's spectrum is closely matched by the high-pressure xenon lamp, with the exception of the lamp's strong emission lines in the near-infrared and some excess ultraviolet. The minimal filtering required and the high efficiency of the xenon lamp mean that both the spectrum and total power of the sun can be achieved in nearly collimated beams over usably-sized areas for laboratory work.
The prior art device of FIG. 1 is typical of the existing or conventional solar simulators 10. The light source 12 is shown as three high intensity lamps 14 of the xenon type. Parabolic or elliptical light collectors 14 surround each of the lamps and direct the light form their respective lamps to a folding mirror 16. The light from each of the separate folding mirrors is directed to a common light collecting mirror 18. The combined light from the mirror 18 is directed through a field lens system 19 which reduces the lens exit diameter for focusing the reduced diameter light beam through an optical integrator lens 20 which provides an expanding light field or projection of the light entering the vacuum chamber. A parabolic output collimator 24 collimates the light and directs the collimated light on the test specimen 26. In this fashion the simulated sun light of the desired spectrum is directed to the specimen to test the effect thereon. For certain testing either the collimator, folding mirrors, field lens and/or vacuum may be eliminated.
The sun simulators as specifically discussed above and including other state of the art simulators are somewhat successful for the purpose intended, but have several features that need improvement. Generally speaking, the state of the art simulators are quite costly to fabricate due to the special lens requirements. They are both difficult to initially align so as to maintain even illumination and to fixedly secure the lens system in place and to maintain the lens system in position. The use of the required number of mirrors and lens produces an inefficient use of primary power, i.e. the best state of the art solar simulators are less than 5% efficient.
The present invention improves the many short comings of the state of the art sun simulators.